Question

While determining the time period of a simple pendulum by the time for twenty or more oscillation is found

Answer 1

It’s because it’s a judgement call when one period finishes and the next begins. Best practice is to set up some visual reference in the middle of the swing and time when the pendulum passes it, because that’s well-defined in time and easy to see coming. By contrast if you count from peak to peak, it’s much less obvious when the peak has been reached. Either way, there’s some reaction time error there, but it’s independent of the number of cycles, so if you time over 20 cycles, you reduce the fractional error by 20.